The preparation of amides has conventionally consisted of reacting an amine with a carboxylic acid, anhydride or acid chloride. Many methods have been described for such amide production. See, e.g., March, Advanced Organic Chemistry, (3d ed., John Wiley and Sons, Inc., 1985) at 1152; Beckwith, The Chemistry of Formamides, (Zabicky, ed., London Interscience Publishers, 1970) at 73; and Sandler & Karo, Organic Functional Group Preparations, (Academic Press, 1968) at 269. Such methods, however, are generally conducted under extreme heat conditions requiring high energy consumption and producing unsuitable by-product formation. Isolation of the desired amide using these methods is, therefore, difficult. The amide yield in the prior art methods is, in many cases, poor and waste disposal of the by-products is costly and ecologically threatening.
Preparations of an amide by reacting a carboxylic acid with an aryl isocyanate date back to the turn of the century. Such preparations, however, have proven essentially ineffective because of the amide/urea product mixtures thereby obtained. Moreover, an isocyanate reacts generally with a carboxylic acid yielding a mixed carbamic carboxylic anhydride. These anhydrides are relatively unstable and decompose. The resulting mixture is a combination of amide, sym-substituted urea and carboxylic anhydride with an evolution of carbon dioxide. Such unstable mixtures may further dissociate into the starting reagents: isocyanate and carboxylic acid. See Mann and Bruist, Ber. Dtsch. Chem. Ges., 309:3052 (1906).
In Haller, Complies Rendus, 121:189 (1895), id. 120:1326 (1895), and id. 64:1326 (1892) it was found that in the reaction of phenyl isocyanate with various carboxylic acids, a mixture of acid anhydride, sym-urea and acid amide were formed. Heating the acid anhydride and the sym-urea to 150.degree. C. or above, generated the acid amide. Vaegeli and Tyabji, Helv. Chim Acta., 18:142 (1935), id. 17:931 (1934), and id. 16:349 (1933) studied the reaction of substituted aromatic isocyanates with carboxylic acids and successfully isolated the mixed anhydride intermediate. They determined that the criteria for mixed anhydride stability was electron withdrawing substitution on the aryl isocyanate. They proposed that decomposition to urea/anhydride occurred through bimolecular disproportion of the mixed anhydride to carbamic anhydride and carboxylic anhydride followed by carbamic anhydride intramolecular rearrangement and CO.sub.2 elimination. Amide formation was suggested to occur through an intramolecular rearrangement of the mixed anhydride with CO.sub.2 elimination.
Fry, J. Am. Chem. Soc., 75:2686 (1952), Troparevski, et al., Aneles Asoc. Quim. Argentina, 61:227 (1973) and Osaki and Shimada, Kogyo Koga Ku Zasshi, 80:506 (1959), confirmed that the aromatic isocyanae was the source of the carbon dioxide when forming either amide or urea and anhydride. They determined that the mixed anhydride would decompose to a urea/anhydride formation through bimolecular disproportion to carbamic anhydride and carboxylic anhydride followed by carbamic anhydride intramolecular rearrangement and carbon dioxide elimination. Further, Osaki and Shimada found that the yield of urea/anhydride was increased with increasing electron withdrawing substitution, increasing temperature, introducing ortho substitution, adding a catalyst, and lower carboxylic acid activity. It was further suggested that pure amide would only be formed at temperatures below -70.degree. C.
An isocyanate/carboxylic acid reaction is typified by U.S. Pat. No. 4,417,002, issued to Liessem. Liessem describes forming a foam material in the presence of a blowing agent where formic acid or its salt is reacted with an isocyanate to liberate gas. Liessem disclosed no product distribution or product structure. In addition, U.S. Pat. No. 4,105,686, issued to Raes, describes the use of a carboxylic acid to deactivate a toluene diisocyanate distillation residue to an inert granular solid. Raes did not discuss product distribution or structure. Moreover, reaction temperatures were on the order of 120.degree. C. to 200.degree. C.
The effect of catalysis has been studied in relation to reactions of aromatic isocyanates with carboxylic acids. For example, S. Ozaki et al., Kogyo Koga Ku Zasshi, at 80:434 (1959), described the catalytic effect of several compounds, including boron trifluoride etherate, on the reaction of phenyl isocyanate with various carboxylic acids. Several different catalysts, including boron trifluoride etherate, increased the reaction rate, but the product distribution remained the same as without any catalyst. Boron trifluoride showed little catalytic effect.
Sarokin, et al., Ko. TR. Mosk. Khim. Technol. Inst. 86:25 (1975) found that catalytic activity in tertiary amine catalysis increased with increasing basicity. Tributylamine and triethylene diamine were noted as exceptions. The same authors presented a paper at the Eleventh Scientific-Technical Conference of Young Scientist and demonstrated that metal catalysts were faster than tertiary amines with a slight preference toward amide formation.
Other conventional methods of reacting aryl isocyanates and carboxylic acids under catalysis include Nikonova and Shoshtaiva, Vspenen Plast. Massy, at 115 (1976), where the reaction of phenyl isocyanate with dicarboxylic acids with and without catalysts was investigated. A mixture of urea and amide resulted, dependent on catalyst use. In addition, U.S. Pat. Nos. 4,061,622; 4,094,866; and 4,156,065, issued to Onder, disclose preparing polyamides from aryl diisocyanates and carboxylic acids, using alkoxy metal salts, alkali metal lactamates and hydrocarbylimino derivatives of phosphorous compounds as catalysts.
Further, U.S. Pat. No. 4,395,531, issued to Toyoda et al., describes the preparation of polyamides from aryl diisocyanates and polycarboxylic acids using at least one mono-alkali metal salt of dicarboxylic acid. U.S Pat. Nos. 4,548,970 and 4,549,006, issued to Zechner et al., describe the preparation of polyamide imides from lactams or polyamides and polyisocyanates and anhydrides using a lactam as an additive.
The prior art methods, however, typically require reaction temperatures in excess of 100.degree. C. to convert urea by-products and the anhydride intermediate to the amide. The catalysts of the prior art are generally ineffective below 100.degree. C. In addition, the prior art methods generally require a two-step procedure: formation of the anhydride and urea, followed by the dehydration reaction of these two intermediates. Low yields and complex product mixtures usually result. Moreover, the catalysts of the prior art are difficult to handle, expensive and poorly efficient in leading to a pure amide product.
In view of the serious deficiencies and inefficiencies of the prior art, it would be desirable to have a method to produce N-aryl amides efficiently, cheaply, easily and with little or no by-product formation.